Except for special cases, the structure of a carbon nanotube (hereinafter may be abbreviated as “CNT”) is often classified into a single-walled CNT having a structure in which only one layer of graphite is wound in a tubular shape, a double-walled CNT in which two layers of graphite form a tubular structure, and a multi-walled CNT in which a plurality of layers of graphite of three or more layers form a concentric cylindrical tubular structure. CNT has attracted attention for more than 10 years as a nanomaterial having excellent mechanical properties, excellent thermal conductivity, and excellent electrical conductivity. Highly pure CNT, however, is difficult to produce in high productivity. In many cases, a low quality CNT is synthesized in large quantities or an inefficient synthetic method that can produce a remarkably highly pure CNT, but cannot produce a sufficient amount of CNT for commercial use is used.
In recent years, it has been announced that a synthesis method called a super growth method (formerly the National Institute of Advanced Industrial Science and Technology) can produce carbon nanotube aggregates (here, a state in which countless CNTs including impurities and additives are collected is determined as the carbon nanotube aggregates) having high purity and low impurities in large quantities. The super growth method refers to a method in which the yield of CNT per process is significantly increased by adding water to oxidize carbon impurities and thus preventing occurrences of the situation where a catalyst surface is covered with the carbon impurities during the synthesis to inactivate the catalyst. The added water, however, may provide damage for not only the carbon impurities, but also CNT. In addition, when production efficiency is considered, it is also a concern that the super growth method is a synthesis method based on a substrate method.
On the other hand, a floating catalyst method has been known as a highly efficient synthesis method in which raw materials are continuously charged and CNT can be continuously recovered. A method of directly and continuously producing a fiber (thread) made of CNT by utilizing the continuity of the floating catalyst method has been developed (refer to Japanese Patent Application Laid-open No. 2012-46841). According to the method described in Japanese Patent Application Laid-open No. 2012-46841, however, twisted yarns of CNTs are ultrafine fibers and only fibers that are difficult to use for applications requiring robustness can be synthesized because the yield of CNT per unit time is remarkably low. Although the yield is remarkably low, a method in which remarkably highly pure carbon nanotube aggregates can be synthesized has also been developed (refer to Japanese Patent Application Laid-open No. 2013-35750). In addition, methods of synthesizing carbon nanotube aggregates including a double-walled CNT by devising two or more of introduced raw materials and devising a mixture ratio of catalysts and the like have also been developed (refer to Japanese Patent Application Laid-open No. 2015-48263 and Japanese Patent Application Laid-open No. 2006-45057).
The methods of producing carbon nanotube aggregates include many production methods characterized in yield, efficiency, purity, crystallinity, number of layers, and the like. Even if different methods or synthesis conditions are combined, however, synergistic effects do not always appear. Consequently, at present, a method of producing carbon nanotube aggregates having high crystallinity and high purity in high efficiency and high yield cannot be practically achieved.
It could therefore be helpful to provide a method of producing a carbon nanotube-containing composition from which carbon nanotube aggregates having high purity can be synthesized in high efficiency and high yield.